I never said they were told by emergency services on the way to the call. I don't know details of what was said in the actual call other than they had awareness some sort of disturbance had taken place.
The point is that when they arrived at the scene, probably without any knowledge at that point of what's gone on, they were told by several people what had happened. That turned out to be a huge, awful, pack of lies. But at the time, there'd have been a natural human instinct to believe the little information you've been given and act based on that. With no mention of a stabbing from the family and information that the "assailant" has fell when trying to run away, as bad as it sounds it isn't irrational to assume he's a suspect who's either drunk and/or concussed saying they've been stabbed when in actual fact they've assaulted someone and fell.